This invention relates generally to coating of substrates using vacuum arc evaporation and more particularly to an improved method and apparatus that employs a cylindrical metallic cathode and a magnetic field to force the motion of an arc into an open helical trajectory on the surface of the cylindrical cathode and to control the speed of movement of the arc. Vacuum arc evaporation has in the last decade or so come into wide commercial use for deposition of metal, alloy, and metal compound coatings. A cathode composed of the material to be deposited is vaporized by a high current, low voltage arc plasma discharge in a vacuum chamber which has been evacuated to a pressure of typically 10.sup.-4 Torr or less. The substrates to be coated are placed in the vacuum chamber facing the evaporable surface of the cathode, at a distance of typically 10-50 cm. Typical arc currents range between 25 and 500 amperes, with voltages between 15 and 50 volts.
The arc plasma discharge conducts electric current between two electrodes in the vacuum chamber, through the metal vapor plasma created by vaporization and ionization of the cathode material by the arc. The cathode, or negative electrode, is an electrically isolated structure which is at least partially consumed during the process. The anode, or positive electrode, may be an electrically isolated structure within the vacuum chamber, or it may be the metal vacuum chamber itself, and is not consumed in the process. An arc is struck on the evaporable surface of the cathode by means of mechanical contact, high voltage spark, or laser irradiation. The ensuing arc plasma discharge is highly localized in one or more mobile arc spots on the cathode surface, but is distributed over a large area at the anode. The extremely high current density in the arc spot, estimated to be 10.sup.6 -10.sup.8 amperes/cm.sup.2, results in local heating, evaporation, and ionization of the cathode material. Each arc spot emits a jet of metal vapor plasma in a direction approximately perpendicular to the cathode surface, forming a luminous plume extending into the region between the cathode and anode, where the substrates to be coated are disposed. The metal vapor condenses on the substrate surface, forming a dense coating. Reactive gases may be introduced into the vacuum chamber during the evaporation process, resulting in the formation of metal compound coatings on the substrate surface.
Below 70-100 amperes of arc current, depending on the material, only a single arc spot will exist. At higher arc currents, multiple arc spots exist simultaneously, each carrying an equal fraction of the total arc current. An arc spot, in the absence of applied magnetic fields, tends to move rapidly and semi-randomly around the target surface, leaving a trail of microscopic crater-like features on the target surface. Although the small-scale motion of the arc is a semi-random jumping from crater site to crater site, the electromagnetic force due to the interaction between the current in the arc jet and any magnetic field present at the cathode surface has a dominant influence on the large-scale average movement of the arc spot. An externally applied magnetic field causes a force on the arc jet in a direction perpendicular to both the field lines and the jet. In the absence of an applied magnetic field, the interaction of the current in the arc jet with the self magnetic field due to the arc current flowing through the cathode can tend to draw the arc spot toward the current input, if the current flow through the cathode is asymmetrical. It is interesting to note that the direction of the motion of the arc in a magnetic field is opposite or retrograde to the vector J.times.B direction expected based on Ampere's law, considering the current to be in the same direction as in the external circuit. This phenomenon has been widely reported and discussed, and is believed to be caused by complex dynamic effects within the arc jet.
An undesirable side effect of the vaporization of target material at the arc spot is the generation of droplets of molten target material, which are ejected from the target by the reaction forces due to expansion of the vapor jet. These droplets are called macroparticles, and range in diameter from sub-micron to tens of microns. The macroparticles become imbedded in the coating when they land on the substrate, forming objectionable irregularities. Various strategies have been devised to reduce generation of macroparticles or prevent their arrival at the substrate.
Several techniques are known in the prior art for depositing a coating onto a substrate. U.S. Pat. No. 2,972,695 describes a magnetically stabilized vacuum arc evaporation apparatus. U.S. Pat. Nos. 3,625,848 and 3,836,451 describe an arc evaporation apparatus with particular electrode configurations and also teach the use of a magnetic field to increase the evaporation rate and to direct ions to the substrate. U.S. Pat. Nos. 3,793,179 and 3,783,21 describe particular configurations of electrodes and shields, as well as the use of a magnetic field activated whenever the arc spot moves off the desired evaporation surface of the cathode.
U.S. Pat. Nos. 4,724,058, 4,673,477, and 4,849,088 describe arc evaporation apparatus using a magnetic field in the shape of a closed loop tunnel, which confines the arc spot to a closed loop "racetrack" trajectory at a fixed location on the cathode surface. The means required to generate such a magnetic field are widely known in the art of planar magnetron sputtering. In order to uniformly erode the entire target surface, it is necessary to move the magnetic field generating means to move the arc trajectory over the target surface, either by mechanical movement of the magnet means as described in U.S. Pat. Nos. 4,673,477 and 4,849,088, or by use of multiple electromagnets, as described in U.S. Pat. No. 4,724,058.
U.S. Pat. Nos. 4,609,564, 4,859,489, and 5,037,522 describe the use of a cathode in the form of a long cylinder or rod, which makes use of the self magnetic field of the arc current to force motion of the arc along the length of the cathode. None of these prior art references shows or suggests a magnetic field means to generate an axial magnetic field component over the cathode surface in order to force the arc to rotate around the cathode as it travels down the length, nor do these references teach any means to increase or control the speed of the arc. It is disclosed in U.S. Pat. No. 5,037,522 that the direction of travel of the arc on the cathode may be reversed by switching the power supply connection from one end of the cathode to the other.
U.S. Pat. No. 4,492,845 describes an arc evaporation apparatus using an annular cathode, in which the evaporable surface is the outer wall, facing a cylindrical anode of larger diameter and greater length than the cathode. The substrates to be coated are disposed inside the annular cathode, not facing the evaporable surface. A coaxial magnetic field is described for diverting charged particles of evaporated material away from the anode and back toward the substrate to be coated.
In accordance with the illustrated preferred embodiments of the present invention, a means is provided to control the speed and path of motion of an arc discharge on a cathode having substantially cylindrical shape, especially a long rod or tube, by application of a magnetic field in the region of the cathode. By controlling the motion of the arc, it is possible to achieve more uniform erosion of the cathode, and hence more uniform deposition onto the substrates than is possible with the random arc movement taught by the prior art. By controlling the speed of the arc, it is possible to minimize generation of macroparticles which roughen the coating.
The motion of an arc on a cylindrical surface may be described as the vector sum of a circumferential component of motion around the circumference of the cylinder, and a longitudinal component motion along its length (parallel to the cylindrical axis). Likewise, the magnetic field of the present invention may be described as the vector sum of circumferential and axial (longitudinal) magnetic field components. Since the force on the arc jet and the magnetic field (the current in the arc jet being primarily perpendicular to the cathode surface), the circumferential component of arc motion is caused by the longitudinal component of the magnetic field, and vice versa.
It is therefore an object of the present invention to superimpose onto the average arc motion a component around the circumference of the cylindrical cathode by application of an axial magnetic field component parallel to the cylindrical axis of the cathode. Under the influence of this axial field component, the arc rotates around the cathode as it travels down the length, the net trajectory forming an open helix.
A further object of the present invention is to control the circumferential velocity, i.e. the speed of rotation, by varying the strength of the applied axial magnetic field component. The magnetic field increases the velocity of the arc motion, thereby reducing macroparticle generation by reducing the amount of time the arc spends in each crater along its track. Experiments have shown that the number and size of macroparticles can be reduced by at least 50% using moderate magnetic fields (1-50 gauss) for at least several cathode materials.
A further object of the present invention is to control the speed and direction of the longitudinal component of the arc motion by controlling the circumferential component of the magnetic field present at the location of the arc spot on the cathode surface. The circumferential magnetic field in the present invention consists of the sum of the self magnetic field due to the arc current flowing through the cathode to the arc spot, and an optional applied circumferential field component due to a variable control current through the cathode, which is supplied independent of the arc current.
If an arc power supply is connected to one end of a rod-shaped cathode in the absence of the applied axial magnetic and circumferential magnetic fields of the present invention, as described in U.S. Pat. No. 4,859,489, and an arc spot is initiated at the end of the cathode opposite from the power supply connection, the arc spot will travel down the length of the cathode under the influence of the self magnetic field of the arc current, in a somewhat random but basically linear path toward the end of the cathode to which the power supply is connected. It is known in the prior art to reverse the direction of travel of the arc along the cathode by switching the power supply connection from one end of the cathode to the other. This method, however, provides no control of the longitudinal speed of the arc motion, since the circumferential magnetic field component which causes the longitudinal motion is due to the arc current, and cannot be varied independent of the arc current.
Control of the strength of the circumferential magnetic field component, and thus control of the longitudinal speed of the arc spot, may be accomplished in accordance with the present invention by either of two methods. In the first method, an arc power supply is connected to both ends of the cathode simultaneously, and a means is provided to control the fraction of the arc current which is supplied to each end of the cathode, while maintaining the total arc current constant between cathode and anode. The net circumferential magnetic field component at the location of the arc spot along the cathode length will then be a function of the division of arc current between the two ends of the cathode. If the current division is balanced, i.e. half the arc current is fed to each end of the cathode, there is no tendency for the arc to move along the length of the cathode, but it will still rotate around the circumference at an independently controllable speed due to the axial magnetic field component of the present invention. If the currents fed to the ends of the cathode are unbalanced, then the arc will move along the length of the cathode toward whichever end is receiving more current, at a speed proportional to the degree of imbalance in the current feed. The arc may thus be forced to rotate around the cathode at a predetermined speed, and be scanned slowly from one end of the cathode to the other at a speed independent of the total arc current, which may be maintained at a constant value.
In the second method of the present invention for controlling the strength of the circumferential magnetic field component, the negative output of an arc power supply is connected to both ends of the cathode simultaneously, such that substantially equal arc current flows to each end, and a circumferential magnetic field component is created over the cathode surface which is independent of the arc current flowing between cathode and anode. This independent circumferential field can be created by connecting an additional power supply to the two ends of the cathode to pass a control current through the cathode from one end to the other, with means provided to adjust the magnitude and polarity of the control current. When this control current through the cathode is zero, the arc has no tendency to move along the length of the cathode, since the net circumferential field is zero due to the balanced manner in which the arc current is supplied to both ends of the cathode. To force the arc to move in one direction or the other along the cathode, a circumferential field is applied at the cathode surface by passing control current through the cathode in the appropriate direction. Since the control current power supply is not part of the cathode-anode circuit, changes in the magnitude or polarity of the control current have no effect on the arc current, which remains constant. This method for control of the longitudinal arc motion has the advantage that the strength of the circumferential magnetic field component can be made greater than the field which would exist from the arc current along. This allows the arc velocity alone the cathode surface to be increased, reducing macroparticle generation, and insures that the arc can be made to move along the cathode length even at low arc currents and in the presence of a strong axial magnetic field component.